Recovery of chemicals in wood pulp preparation



Feb. 18, 1958 K. R. GRA? "ET Al 2824071 RECOVERY OF CHEMICALS IN WOOD PULP PREPARATION K. R. GRAY ET AL Feb. 18, 1958 RECOVERY OF CHEMICALS IN WOOD PULP PREPARATION Filed May 15, 1953 2 Sheets-Sheet 2 ATTORNEYS In order to provide, by means of sodum carbonate, a smelt of suflicient fluidity to be readily recovered frorn conventional smelting furnaces of the type used in the kraft pulping industry, t is necessary te treat the sulfite waste liquor with suflcient sodum carbonate to (1) neu tralize free lignosulfonic acids and free S (2) provide suflicient additional sodum carbonate in the liquor to be equivalent to the S0 resulting trom combustion according to the above equation (ths being equivalent to a smelt of 100% sulfidity) and (3) add suflcient excess sodum carbonate over and above this to produce a smelt containing a large portion of sodum carbonate.

Table I above indicates that optimum results are obtained by having present sufiicient excess sodum carbonate to produce a smelt of about 50% sulfidity. In practice, however, it will frequently be advantageous to add sufiicient sodum carbonate to the waste liquor before furnacing or smelting to produce a smelt of about 2550% sulfidity or even lower. While the reason for this is not clear, it is believed to be due to the fact that in addition to the preponderance of sodum carbonate and sodum sulfide there will always be present smaller amounts of other sodum chemicals such as sodum sulfate, thiosulfate, and, if saltwater floated wood has been used, sodum chloride. These will modify the properties of the smelt.

There could be certain disadvantages in adding aqueons solutions of sodum carbonate to waste liquor for neutralization and sulfidity reduction prior to furnacing. Due to the limited solubility of sodum salts of carbonic acid and the large amount of sodum carbonate required for addition, an excessive burden of water would be added along with the chemical requiring considerable additional evaporation. For example, when using a sodum carbonate solution of 100 grams per liter concentration (which can be readily obtained by carbonating a sodum sulfide-containing smelt in solution) 90 pounds of water would be added for each pounds of carbonate. Thus, it will be seen that it would be a tremendous advantage if the large amount of sodum carbonate to be added to the sulfite waste liquor to obtain the benefits outlined above could be added in solid form.

Our invention provides a simple, practical process for converting the sodum content of soda-base acid sulfite waste liquor into a fluid smelt predominating in sodum carbonate and sodum sulfide, with concomitant recovery of heat and recovery in the smelt of a substantial portion of the sulfur originally present in the waste liquor. The invention .further provides a simple process for con verting this smelt into solid sodum carbonate of such high purity that it may be used for the preparation of acid sulfite digestion liquor, and which is so nearly water tree that it may be used for neutralizing and lowering the sulfidity of the sulfite waste liquor without resnlting uneconomical dilution. Simultaneously, the sulfide content of the smelt is recoVered in the form of concentratecl hydrogen sulfide which can be readily burned to sulfur dioxide of such concentratiori that it can be readily and conveniently used in the preparation of acid sulfite digestion liquor.

The process of the nvention comprises several interrelated cyclic systems for the recovery of soda-base chemicals from a soda-base acid-sulfite pulping operation characterized by the incorporation in the waste liquor of sodum carbonate, which is forrned in the process, to lower the melting point of the smelt to a point where it will flow out of the furnace.

One important feature of the invention is that we form a low melting point smelt of sodum sulfide fluxed with sodum carbonate and we dissolve this smelt in such a limited amount of water that we form a slurry of crystals of sodum carbonate in a concentrated solution of sodum sulfide. Advantageously, we form this slurry from mother liquor returned from the stripping operation 4 wherein hydrogen sulfide is recovered and which can tains these salts in high concentration.

In accorclance with a more complete process of the invention, we separate from the sodum sulfide solution of the slurry a solid portion consisting of crystallized sodum carbonate and smaller amounts of crystalline sodum sulfate formed in the solution, which solid portion is generally divided into two parts, one part of which is added to the Waste liquor to flux the sodum sulfide of the smelt and the other part is used to make t:p "sodabase acid sulfite cooking liquor. The residual sodum sulfide solution from the slurry is subjected to a pressure carbonation with carbon dioxide, followed by vacuum strpping to release concentratecl hydrogen sulfide and form in solution sodum carbonate. The CIICII trated hyrlrogcn sulfide which readily supports combus tion is burned to form sulfur dioxide which is reacted with one part of the separated crystalline sodum carbonate to prepare soda-base acid sulfite digestion liquor for return to the pulping operation. In the carbonation and stripping operation, the solution of sodum sulfide is treated with carbon dioxide under a pressure of at least 16 pounds per square inch absolute, preferably at a temperature of trom 50 to C. and the solution is then stripped under a vacuum of from 5 to 29 inches of mercury, as described in our copending application Serial No. 166,683, filed June 7, 1950, now abancloned, which subject matter is incorporated in application Serial No. 415,754, filed March 12, 1954.

While we ordinarily find it sufiicient to subject the sodum sulfide solution to a single stage of pressure carbonation followed by vaeuum stripping, we may use a plurality of pressure carbonations, each followed by vacuum stripping as described in our copending application, Serial No. 166,681, filed June 7, 1950, now abandoned, which subiect matter is incorporated in application Serial No. 415,817, filed March 12, 1954.

In a particularly advantageous operation of the invention, the waste liquor from the soda-base acid sulfite digestion of wood chips is evaporated and burned under reducng conditions to form a free-flowing smelt which is partially dissolved in such a limited amount of water, preferably a mother liquor from the stripping operation containing sodum carbonate and sodum sulfide, to form a concentrated solution of sodum sulfide in a slurry of sodum carbonate and sodum sulfate, which concentrated solution after removal of the crystals is subjected to carbonation followed by stripping to remove hydrogen sulfide as aforesaid. The operation of the invention pro vides at least four interdependent cyclic systems comprising: (a) circulating the stripped liquor to the smelt dissolving tank to form a, concentrated solution of sodum sulfide containing sodum carbonate crystals as a slurry, (b) separating part of the sodum carbonate crystals and dlrecting them to the waste liquor prior to furnacing as a flux to lower the melting temperature of the smelt, (c)

directing the other part of the sodum carbonate crystals to a soda-base acid sulfite liquor make-up operation, and (d) subjecting the sodum sulfide solution to p1essure carbonation and vacuum stripping to release concentrated hydrogen sulfide, then oxidizing the hydrogen sulfide to sulfur dioxide and passing the sulfur dioxide to the sodabase acid sulfite liquor make-up operation and eventually returning the make-up soda-base acid sulfite liquor to the digestion operation.

Ourinvention gives the important advantage of: a low melting point smelt that is fiuxed with sodum carbonate prepared in the operation and of a concentrated solution of sodum sulfide from which the sulfur may be recovered eflciently and in high concentration for return to the operation to form make-up liquor. Other advantages will be apparentfrom the following discussion.

In the accompanying drawings:

Fig. 1 is a diagrammatic; llustration of the four im pcftant nterdependent cyclic systems in a process of the invention, and

Fig. 2 is a flow-sheet of an arrangement of apparatus suitable for carrying out an operation in accordance with the process of the invention.

The aforementioned four interdependent cyclic systems of the improved process are illustrated generally in Fig. 1. The original waste liquor from the wood digestion, known as sulfite waste liquor (SWL), if evaporated and burned would form a high melting point smelt that would be difficult to remove from the smelting furnace. In our process, the melting point of the smelt is lowered by add ir1g solid sodium carbonatefrom the slurry to the sulfite waste liquor by cycle 1. The resulting low melting point smelt is dissolved in a minimum amount of water, advantageously by adding stripped mother liquor from cycle 2 to form a slurry of sodium carbonate crystals in a con centrated solution of sodium sulfide. By using a limted amount of water at this point, and especally by using the stripped mother liquor which is a solution already rich in sodium carbonate and sodiumsulfide, we effect apractical separation of crystals of sodium carbonate for use in the process and also obtain a concentrated solution of: sodium sulfide from which sulfur is recovered. The ether part of the sodium carbonate crystals is sent to the acid sulfite digestion liquor make-up by cycle 3 where it is reacted with sulfur dioxide from cycle 4 to form sodabase acid sulfite liquor for return to the digester. It will be apparent that the process comprises four interdependent cyclic systems which contribute to an efiicient operation for the recovery of soda-base chemicals in the digestion of wood by the soda-base acidsulfite process.

In carrying out an operaton in an arrangement of apparatus such as that illustrated in Fig. 2, the wood chips are subjected to any suitable digestion with soda-base acid sulfite cooking liquor in the digester 1. The waste liquor from the digestion is treated at 2 with solidsodium carbonate releasng carbon dioxide which may be used inthe carbonation and the treated liquor may be clarified at 3 for the removal of sludge. The treated liquor with or without such clarification is concentrated -by evaporation, preferably to at least 50% total solids content as in multiple-effect evaporators 4 (or in multiple-effect evaporators followed by evaporation in direct contact with flue gas as in cascade or disc evaporators), and the concentrate is burned in the smelting furnace 5. This furnace may be similar to the conventional heat and chemical recovery units now used in the kraft and soda pulp mills.

The furnace is operated under reducing conditions and the resulting sodium sulfide, fluxed with sodium carbonate, produces a low melting point smelt that flows out of the furnace to the slurry tank 6. The low sullidity of the smelt tends to minimize any possibility of explosions resulting trom adding the molten smelt to the dissolving liquor (which may be water 01 preferably mother liquor from the stripper). This liquor is quite concentratecl and may contain solids consisting, for example, of about 70% sodium carbonate and 30% sodium sulfide.

Alternatively, the smelt after issuing from the furnace may be cooled and solidified and then added to the dissolvihg liquor or water in such form. The cooling would bviate the riecessity of adding excess water to dissipate the sensible heat of hot molten smelt as steam. In any event only a limited amount of water is added to the smelt (as an example, to 1 part of smelt 1 part of water after allowing for any evaporation), so that substantially all the sodium sulfide is dissolved without appreciably dissolving the sodium carbonate. A certain amount of sodium sulfate will be present in the smelt due to incomplete reduction and will largely crystallize out along with thepreponderant amount of sodium carbonate to form a slurry in a concentrated solution of sodium sulfide. The slurry is filtered in the filter or clarifier 7, the solution of sodium slfide being sent to the carbonator 8 together with Some added water and the solids of the slurry,

namely, sodium carbonate and sodium sulfate, are dvided into two parts. One part is mixed with the sulfite waste liquor tor neutralization and sulfidity reduction as aforesaid, while the other part is washed at 9 with water or with wash water containing some sodium carbonate, sodium sulfate, and the washings containing sodium sulfide are returned to the filter while the washed crystals of sodium carbonate and sodium sultate are dissolved in water at the acid-makeup unit 11 for combination with sulfur doxide to form soda-base acid sulfite liquor which is passed to the digester 1 for the digestion of the chips.

The concentraied solution of sodium sulfide from the filter 7, which may have a sulfidity of around is passed into the pressure carbonation charnber 8 and then into the vacuum stripping unit 12 for the recovery of hydrogen sulide and the formation of a mother liquor of reduced sulfidity, e. g. containing solids in the ratio of around 70% of sodium carbonate and 30% sodium sulfide. This single stage carbonation and stripping may be carried out as described in our copending application, Serial No. 415,754.

The stripping operation may be carried out in any type of apparatus commonly used for strpping such as a packed tower and under a vacuum of from 5 to 29 inches of mercury. In that it follows a pressure carbonation a flashing is involved, and a large porportion of the hydrogen sulfide is evolved in highly concentrated form with but little dilution with carbon dioxide resulting frorn decomposition of bicarbonate.

In the preferred form of our invention, the sulfidecontaining solution is subjected to a carbonation treatment in which the solution is treated preferably in a countercurrent marmer, ander pressure with a gas containing carbon dioxide, such carbonation treatment being followed by steam stripping onder a vacuum of from 5 to 29 inches of mercury, preferably at least 20 inches of mercury, to remove volatile hydrogen sulfide in concentrted form, whereby the sulfide content of the smelt solution is substantially lowered. In ths preferred method of operation, the carbonation treatment is generally eflected at a temperature of 50150 C. with a gas pressure of 20-165 pounds per square inch absolute, such that the mol ratio of carbon dioxide absorbed to total titratable alkali in the solution is in the range of 0.6 te 1.5. In that carbonation is carried out at an elevated temperature and that sodium carbonate is consumed in the reaction with sodium hydrosulfide in the stripping operation, relatively concentrated solutions of soda salts (e. g. of the order of gm. per liter as Na O) may be treated without the formation of precpitates durr1g processing.

By total titratable alkali in the above is meant the basicity equivalent to a standard acid titration to the methyl orange end point. In the case of soda smelts, this would nclude all of the sulfide, carbonate, and caustic soda, and ome-half of the sulfite, and would exclude such salts as thiosulfate, sulfate, and chloride.

Under the stripping conditions of the invention, i. e. releasing the hot pressure carbonated solution into a vacuum, thus contacting it with steam at low tempera ture, very little bicarbonate decomposition occurs, with apparently reduces carbon dioxide losses from bicarbo nate decomposition.

Irrespective of whether pure CO or flue gas is used for the carbonation, no attempt is made to completely eliminate sulfide in strippng since, as the stripping operaton approaches completion, the eficiency of strip.j

"7 ping Wh1 regard; to steam consumption decreasestre mendously.

The carbonation and stripping operations may be carried out in any. type of equprnent conventionally employed for gas absorption or stripping operations, for example in packed columns, plate columns, spray columns, and contnuous liquid phase columns. Agitated gas disperson equipment mght be advantageously used for the carbonation stage.

Flue gas may be used to advantage, requirng only slightly higher operating pressures or larger equipment than when using purer gas. Where a portion of the solid sodium carbonate produced. is to be used for the preparation of acidsulfite digestion liquor, a high degree of purity of the carbonate is. desirable and t will be advisable to purify the flue gas; by removing. suspended matter and scrubbing, out any sulfur dioxide or other undesirable contaminanh Lime kiln gas, if available, may often be treed trom suspended matter and used to advantage, since it will normally contin from 30% to 45% carbon dioxide.

When usi-ng dilu-ted carbon dioxide (e. g. flue gas or lime kln gas) in the pressure carbonaton, countercurrent flow is advantageously used. Under these condi tiens, the gas leaving the top of the carbonator is in contact with highly alkaline solutions (i. e., smelt solu tions). Probably because of the high alkalinity at the point of gas exit, the amount of hydrogen sulfide leaving the top of the carbonator in dilute form is very low. As. a result, the preponderant portion of the total hydrogen sulfide produced: is liberated in the stripping operations in hghly cncentrated,readily usable form.

Using pure carbon doxide for carbonation, thiosulfate formation is negligible. Using diluted carbon dioxide containing oxygen (e. g., flue gas) some thiosulfate is formed but the amount is surprisingly low and, therefore, will not tend to build up rapidly in the system. The small amount of thiosulfate formed by use of oxygen-containing flue gas in the solution carbonation process of our invention is in striking contrast to the very large amounts of thiosul-fate produced when sold smelt is carbonated by flue gas.

The solution obtained from this carbonation treatment is introduced into a stripper which is rnaintained at a pressure substantially below the carbonation pressure and below the vapor pressure of water at the temperature of the carbonaton,. whereby aflashing or sudden release of water vapor occurs accompanied by the simultaneousevolution of a portion of the hydrogen sulfidewith only a relatively small proprtion of. carbon dioxide. Although not an essential feature of our process, it has been .found desrable to allow the flashing to occur in the vapor space at the top of a column, whereupon the. remaning liquid passes down through this column, which may be a packed column, plate column, or similar equipment, in contact with ascending steam supplied to the base of the column. This etects the efficient removal of the hydrogen sulfide formed in the process.

The recovered H S from the vacuum. stripper will contain some steam which may be condensed out resulting in.recvery of theH S at ahigh concentration. This 1ay be readily oxidized by burning with airtoform sulfur doxide for use in the preparation of acid sulfite diges tion liquor. This sulfur dioxde is absorbed in a solution of the sodium carbonate from the slurry which is dissolvecl and passed into the unit for cooking liquor make-up to produce soda-base acid sulfite liquor for digestion.

The invention may also be used in connectiou with the process of our copending application Serial No. 415,- 817 and also Letters Patent 2,656244 of Kenneth Russell Gray and .Hartzell Lance Crosby, filed. July 15, 1950.. Where such a combihaton isefi'ected all the:advantages outlinecl above which are obtained by theproducton of solid sodium carbonate for return to the sulfite waste liquor are gained, the only modificatlon being tl1at soda. chemicals required for the acid sulfite digestion liquor are: produced by one of the cited copendng processes.

In one method of carrying out such integration, the smelt fromthe furnace is divided into two portibns. The larger porton will be treatecl by the process of the present invention to produce solidsodi1m carbonate .for addition to the sulfite waste liquor prior to furnacing and to re cover hydrogeh sulfide. The smaller portion of the smelt will be dissolved in water and may be treated by the iorr exchange process of. said Patent 2,656,244 to produce soda-base acid sulfite digestion liquor directly, According. to this process the solution containing this portibn of the smelt is contacted with a carboxylc acid-type or phenolic-type cation exchange resin in hydrogen form whereby sodium is adsorbed on the resin and a. mixture of CO and H S may be evolved. The adsorbed sodium is subsequently desorbed from the resin by a sulfurous acid solution to produce soda base acid sulfite digestion liquor. The aqueous eflluent from this resin treatment containing unadsorbed. cations and anions may he added to the smelt dissolvihg, tank 6 in Fig. 2. Any mixture of CO and H S evolved may be added to the H S evolved from the stripper 12 or may be mixed withthe flue gas or other CO goi-ngto the carbonator 8.

Alternati-vely, the. smaller portion of the smelt after dissolving in water may, be treated by the multi-stage solution process of om. pending application Serial No.- 415,817, to produce a solntion: of sulfide-free sodium carbonate which, by reaction with sulfur dioxide or sulfur dibxide containing gas, may be converted to soda-base acid sulfite digeston liquon In these two methods of integraton where the solidsodim carbonate produced by the present ihventionis used solely for addition to the waste liquor, it is not necessary to purify the flue gas used instep 8 trom suspended solids.

Thus.in the case ofintegratbn with the ion exchange process of said Patent 2,656,244tbr the production -of acid.sulfite digestibn liquor, no purfication of any ofthe flue gas would be required. In: the case ofintcgraton with the multi-stage solution-carbonation process,v Serial would: be needd to tr.eat the smaller portion of the soda chemicalswhich are .to be converted by the carbonation acid sulfite digestion liquor or from the neutralization of sulfite waste liquor.

A further method? ofintegrating the invention with the" 1on exchange process of said Patent 2,656,244 for the preparation of acid sulfite dgestionliquor, nvolves treat ing the mother liquor from. the stripper (step 12, Fig. 2);

by means of. such ion exchange process. This serves to produce a very pure sodium biSulfite-sulfurous acid solution for wood diges-tion We claim: -1. In recoverii1g, chemicals and heat from soda-base acid-sulfite waste liquor, the. improvement which comprses evaporatng the liquor and burning. the concentrate in a f11rnace toform a smelt, adding to the smelt stripped aqueous liquor from the process to form a slurry of sodium carbonatecrystals in a concentrated solution of.

sodium sulfide, separatng.sodum carbonate crystals fromthe slurry, circulatng. part of" the sodium carbonate crystals back to/ the waste liquor to flux the contained chemicals andlo.wer the melti-ng temperature ofthe smelt,

directing the o.ther part of the sodium carbonate to soda;-

base acid-sulfite liquor make-up, subjecting theconcen-- trated sodium. sulfide solution: to carbonation audsteamstripping to:form..hydrogensulfide andsaid stripped aqueons liquor, oxidizing the hydrogen sulfide to sulfur dioxide, reacting the sulfur dioxide with the sodium c8.r-=

bonate to make up the soda-base acid-sulfite liquor, said carbonation being crried out at a temperature above 50 C. under a pressure of at least 16 pounds per square inch absolute and said stripping being carred out under a vacuum of from to 29 inches of mercury.

2. In recoverng chemicals trom soda-base acid-sulfite liquor, the improvement whch comprises evaporating the sulfite waste liquor and burning the resulting concentrate under reducing conditions to form a smelt, forming a concentrated sodum sulfide solution of said smelt and subjecting said solution to carbonation and stripping to form a strpped liquor and hydrogen sulfide, in said operation providing at least four interdependent cyclic systems comprising (1) circulating strpped liquor to the smelt to form the concentrated solution of sodum sulfide and a slurry of sodum carbonate crystals, (2) separatng part of the sodum carbonate crystals and drectng them to the waste liquor as a flux to lower the meltng temperature of the smelt, (3) directing the other part of the sodum carbonate crystals to a soda-base acid-sulfite liquor make-up operation, and (4) oxidizing the hydrogen sulfide to sulfur dioxde and passing the sulfur dioxide to the soda-base acidsulfite liquor make-up operaton.

3. In recovering chemicals from soda-base acid-sulfite liquor, the improvement whch comprises evaporating the sulfite waste liquor and burning the resulting concentrate under reducing conditions to form a smelt containing sodum sulfide and sodum carbonate, adding to the smelt such a limted quantity of water that a concentrated solution of sodum sulfide results and a slurry of sodum carbonate and sodum sulfate crystals form, iucorporating at least a part of the sodum carbonate crystals in the sulfite waste liquor to serve as a flux for the smelt, and subjectng the sodum sulfide solution from the smelt to treatrnent for the recovery of sulfur compound.

4. In recovering chemicals from sodabase acid-sulfite liquor, the improvement whch comprises evaporating the sulfite waste liquor and burnng the resulting concentrate uncler reducing condtions to form a smelt containng sodum sulfide and sodum carbonate, dissolving at least a part of the smelt in such a limited amount of water that a slurry is formed consisting of crystals of sodum carbonate and sodum sulfate in a concentrated solution of sodum sulfide, incorporating sorne of the sodum carbonate crystals from the slurry in the sulfite Waste liquor to flux the smelt, subjecting the solution of sodum sulfide to carbonation with carbon doxide with out appreciably releasing hydrogen sulfide, and then subjecting the solution following carbonation to stripping under a vacuum to recover hydrogen sulfide.

5. In recovering chemicals from soda-base acid-sulfite waste liquor, the improvernent whch comprises evaporating the sulfite waste liquor and burning the resulting concentrate under reducing conditions to form a fiuid smelt, removing the fluid smelt for cooling and solidificaton, addng only a suficient amount of water to the sold smelt to dissolve the sodum sulfide without appreciably dssolving the sodum carbonate to form a slurry in a concentrated solution of sodum sulfide, separatng the solution of sodum sulfide from the slurry and subjecting the solution to carbonation and strippng, divding the sold portion of the slurry comprising sodum carbonate crystals into two parts, incorporating one part in the sulfite waste liquor for neutralization and sulfidity reduction and to serve as a flux for the smelt produced therefrom, and reacting the sodum carbonate of the other part with sulfur dioxide to form soda-base acd-sulfite liquor.

6. In recovering chemicals and heat from soda-base acid sulfite waste liquor, the improvement whch comprises evaporating and burnng the concentrate in a furnace to form a smelt, dividing the smelt into two parts, adding to one part of the smelt stripped aqueous liquor from the process to form a slurry of sodum carbonate crystals in a concentrated solution of sodum sulfide,

separating the sodum carbonate crystals from the slurry and adding them to the sulfite waste liquor prior to furnacing to serve as a flux for the smelt, subjectng the sodum sulfide solution to carbonation and steam stripping to form hydrogen sulfide and said stripped aqueous liquor, oxdizng the hydrogen sulfide to sulfur dioxide, dissolving the other part of the smelt in water, contacting this solution with a weakly acidic cation exchange resin from the class consstng of carboxylic acid-type and phenolic-type cation exchange resin in the hydrogen state whereby sodum is adsorbed, and desorbng the adsorbed sodum with a solution of the said sulfur dioxide to produce soda-base acid sulfite digestion liquor.

7. In recoverng chemicals and heat from soda-base acid sulfite waste liquor, the improvernent whch comprises evaporating and burning the concentrate in a furnace to form a smelt, adding to the smelt stripped aqueous liquor from the process to form a slurry of sodum carbonate crystals in a concentrated solution of sodum sulfide, separating the sodum carbonate crystals from the slurry, and addng them to the sulfitewaste liqucr prior to furnacing to serve as a flux for the smelt, subjecting the sodum sulfide solution to carbonation and steam stripping to form hydrogen sulfide and stripped aqueous liquor, oxidizing the hydrogen sulfide to sulfur dioxide, dividing the stripped aqueous liquor into two parts and adding one part to the said smelt, contacting the other part of the stripped aqueous liquor with a weakly acidic cation exchange resin selected from the class conssting of a carboxylic acid-type and a phenolic-type cation exchange resin in the hydrogen state whereby sodum is adsorbecl, and desorbing the adsorbed sodum with a solution of the said sulfur dioxide to produce soda-base acid sulfite dgestion liquor.

8. In recovering chemicals and heat trom soda-base acid sulfite waste liquor, the improvement whch comprises evaporating and burnng the concentrate in a furnace to form a smelt, adding to the smelt stripped aqueous liquor from the process to form a slurry of sodum carbonate crystals in a concentrated solution of sodum sulfide, separating the sodum carbonate crystals from the slurry and adding them to the sulfite waste liquor prior to furnacing to serve as a flux for the smelt, subjecting the sodum sulfide solution to carbonation and steam strippng t0 form hydrogen sulfide and stripped aqueous liquor containng sodum carbonate and sodum sulfide, oxidizing the hydrogen sulfide to sulfur dioxide, dviding the stripped aqueous liquor into two parts and adding one part to the said smelt, subjecting the other part of the stripped aqueous liquor to additional carbonaton and steam strippng to produce a solution of sulfide-tree sodum carbonate, and absorbng said sulfur dioxde in this solution of sulfide-free sodum carbonate to produce soda-base acid sulfite digeston liquor.

9. In recovering chemicals and heat from soda-base acid-sulfite waste liquor, the improvement whch comprises evaporating the liquor and burning the concentrate in a furnace to form a smelt, adding to at least a portion of the smelt stripped aqueous liquor from the process to ferm a slurry of sodum carbonate crystals in a concentrated solution of sodum sulfide, separating sodum carbonate crystals from the slurry, circulating at least a part of the sodum carbonate crystals back to the waste liquor to flux the contaned chemicals and lower the melting temperature of the smelt, subjecting the concentrated sodum sulfide solution to carbonation and steam stripping t0 form hydrogen sulfide and said stripped aqueous liquor, said carbonation being carried out at a temperature above 50 C. under a pressure of at least 16 pounds per square inch absolute and said stripping being carred out under a vacuum of from 5 to 29 inches of mercury.

(References 0u following page) References Cited in the file of this patent UNITED STA-TBS PATENTS Richter .une 28, 1932 Richter Apr. 18, 1933 5 Bradley et al. Nov. 7, 1933 Bradley et al. Dec. 11, 1934 

1. IN RECOVERING CHEMICALS AND HEAT FROM SODA-BASE ACID-SULFITE WASTE LIQUOR, THE IMPROVEMENT WHICH COMPRISES EVAPORATING THE LIQUOR AND BURNING THE CONCENTRATE IN A FURNACE TO FORM A SMELT, ADDING TO THE SMELT STRIPPED AQUEOUS LIQUOR FROM THE PROCESS TO FORM A SLURRY OF SODIUM CARBONATE CRYSTALS IN A CONCENTRATED SOLUTION OF SODIUM SULFIDE, SEPARATING SODIUM CARBONATE CRYSTALS FROM THE SLURRY, CIRCULATING PART OF THE SODIUM CARBONATE CRYSTALS BACK TO THE WASTE LIQUOR TO FLUX THE CONTAINED CHEMICALS AND LOWER THE MELTING TEMPERATURE OF THE SMELT, DIRECTING THE OTHER PART OF THE SODIUM CARBONATE TO SODABASE ACID-SULFITE LIQUOR MAKE-UP, SUBJECTING THE CONCENTRATED SODIUM SULFIDE SOLUTION TO CARBONATION AND STEAM STRIPPING TO FORM HYDROGEN SULFIDE AND SAID STRIPPED AQUEOUS LIQUOR, OXIDIZING THE HYDROGEN SULFIDE TO SULFUR DIOXIDE, REACTING THE SULFUR DIOXIDE WITH THE SODIUM CARBONATE TO MAKE UP THE SODA-BASE ACID-SULFITE LIQUOR, SAID CARBONATION BEING CARRIED OUT AT A TEMPERATURE ABOVE 50*C. UNDER A PRESSURE OF AT ALEST 16 POUNDS PER SQUARE INCH ABSOLUTE AND SAID STRIPPING BEING CARRIED OUT UNDER A VACUUM OF FROM 5 TO 29 INCHES OF MERCURY. 